This invention relates to presses, more particularly for manually-operated presses adapted for use in bending, shearing, and notching thin metal workpieces, such as the thin metal creasing, cutting, or perforating rule used in the box manufacturing industry.
Manually-operated presses have been in existence for over a century for use in a wide variety of fabrication processes. Although hydraulic or other power-assisted presses have replaced manually-operated presses for heavy fabrication operations, manually-operated presses are still the most economical choice for fabrication of relatively thin or soft materials. Where the fabrication process requires a relatively constant force, especially over a relatively long stroke, a simple arbor press is frequently used. Where the fabrication process requires greater force multiplication than possible with an arbor press, a press incorporating some sort of toggle mechanism is frequently employed. A toggle mechanism generates an output force proportional to the reciprocal of the sine of the angle measured between the toggle links and the line of action of the output force. Accordingly, the theoretical output of a toggle mechanism asymptotically approaches infinity as the toggle links approach a straight line. This makes a toggle-actuated press ideal for shearing or cutting workpieces having thin cross-sections because all of the work is done near the end of the stroke when the toggle link angles approach zero and the mechanical advantage is highest. Accordingly, toggle link actuated presses are the preferred embodiment for use in bending and notching the thin metal rule used in the box manufacturing industry. Typical rule bending and notching presses, such as shown in FIG. 1 marked "prior art" are modestly-sized table-mounted presses with horizontal rams and an operating handle projecting generally horizontally to allow a single operator to manipulate the workpiece and operate the press simultaneously.
Use of a toggle mechanism in the design of a press, however, generally requires trade-offs between stroke and mechanical advantage. If a toggle mechanism is used for shearing thicker materials, or for bending operations, where the ram of the press must perform work over a longer distance, operator fatigue is a problem because the mechanical advantage of a toggle-actuated press drops off rapidly as the toggle link angles increase. Cam actuated presses can be made with a cam profile tailored to provide an elevated force over a longer stroke, however, cam actuated presses require a secondary mechanism to retract the ram, which adds to the cost and complexity and may induce hysteresis between the advancement and retraction of the press ram, thereby detracting from the operator's ability to "feel" the motion of the ram. In a typical rule notching press, the stroke of the ram is on the order of 1 to 11/2 inches. Most of this stroke, however, is solely to allow the dies and knives to be exchanged within the apparatus. The press typically operates in a loaded condition for only about the last 0.100 inch of its stroke. Likewise, rule bending presses typically have a stroke of about 1 to 1.5 inches to accommodated exchanging of dies, however, they typically operate under load for only about the last 1/2 inch of the stroke. Notwithstanding that the loaded stroke of most rule bending and notching presses are less than 1/2 inch, prior art presses are universally designed with a handle stroke of less than about 90.degree., presumably to allow the operator to apply his or her full weight on the handle throughout the full stroke of the ram. Unfortunately, this arrangement limits the maximum mechanical advantage that can be achieved at the end of the stroke, where it is most important. Additionally, prior art rule bending and notching presses generally have fixed dimensions for the linkages and, therefore, cannot easily be adjusted to provide greater or lesser mechanical advantage as needed. What is needed then is a toggle-actuated rule bending or notching press that develops a high output force concentrated over the last 0.100 to 1/2 inch of the stroke, without loss of operating convenience attributable to providing a 1 to 11/2 inch total stroke. What is also needed is a hand-operated press that has an easily adjustable and predictable mechanical advantage.